Antigen presenting cells

Introduction

The population of specialized cells: 

1. to capture microbial and other antigens,
2. display MHC complexes in association with these peptide fragments of protein antigens on its surface to lymphocytes, and
3. provide signals that stimulate the proliferation and differentiation of the lymphocytes (co-stimulatory signal) are known as the antigen-presenting cells (APCs). 

APCs are conventionally referred to as those cells which display antigens on their surface to the T lymphocytes. A dendritic cell is the major type of APC which is involved in initiating the T cell responses. 

Macrophages and B cells also present antigens to the T lymphocytes but in different types of immune responses. The follicular dendritic cell, a specialized cell type, displays antigens to B lymphocytes during particular phases of humoral immune responses. APCs thus link responses of the innate immune system to responses of the adaptive immune system, and therefore they may be considered components of both systems. 

Types of APCs: 

1. Dendritic Cells

Dendritic cells form one of the most important APCs for activating naive T cells. These cells constitutively express a high level of class II MHC molecules and deliver a co-stimulatory activity and thus play major roles in innate responses to infections and link innate and adaptive immune responses.

2. Antigen-Presenting Cells for Effector T -Lymphocytes

In addition to dendritic cells; macrophages and B lymphocytes perform important antigen-presenting functions in CD4+ helper T cell-mediated immune responses. 

1. Macrophages present antigen to helper T lymphocytes at the sites of infection, which leads to helper T cell activation and production of molecules that further activate the macrophages. These macrophages must be activated by phagocytosis of particulate antigens before expressing class II MHC molecules or the co-stimulatory B7 membrane molecule.

2. B cells present antigens to helper T cells in lymph nodes and spleen, a key step in the cooperation of helper T cells with B cells in humoral immune responses to protein antigens. These B cells constitutively express class II MHC molecules but must be activated before expressing the co-stimulatory B7 molecule. 

Note: Cytotoxic T lymphocytes (CTLs) are effector CD8+ T cells that can recognize antigens on any nucleated cell and become activated to kill the cell. Thus, all nucleated cells are potentially APCs for CTLs. 

3. Follicular Dendritic Cells: 

Follicular dendritic cells (FDCs) are cells with membranous projections found intermingled in specialized collections of activated B cells, called germinal centers, in the lymphoid follicles of the lymph nodes, spleen, and mucosal lymphoid tissues. 

Article by-  SAMPRATI PAREKH (MSIWM049)

References:

1. Cellular and Molecular Immunology by Abul K. Abbas – 7^thEdition 
2. Kuby Immunology – 5^thEdition.